As the semiconductor industry continues to increase the density of devices manufacturable on a semiconductor substrate, the device features continue to shrink below the quarter-micron size. The critical dimensions of vias between different metal layers are decreased, making it more difficult to achieve void-free via fill as the stringent restrictions on the pitch and space results in straight-walled vias with high aspect ratios. The problem of having voids become apparent after a chemical mechanical planarization (CMP) step followed by a subsequent wet chemical cleaning step. The voids can potentially open up a pathway for fluids to become entrapped within the via.
A tungsten via fill process that is commonly known to those skilled in the art typically consists of two important film deposition steps. The two film deposition steps include a nucleation step and a bulk film deposition step. The nucleation step is typically associated with poor step coverage, which may eventually lead to voiding (due to premature closure of the via) and increased surface roughness. Further, the nucleation step is unstable with a commonly known process having a 1:1 gas flow rate ratio of tungsten hexafluoride (WF.sub.6) and silane (SiH.sub.4) at a certain pressure regimes, such as approximately 20 to 40 torr. Within this regime, the nucleation step is sensitive to the precision of the actual gas flow rates, especially during the initial stages of nucleation. Mass flow controllers (MFCs), are used to regulate the gas flow rate for a silane gas source and a tungsten gas source. The response times of the MFCs affect the nucleation film. For example, if the MFC for the tungsten gas source responds quicker than the MFC from the silane gas source, the film exhibits localized thinning near the center of the wafer. The currently known process, as a result, requires precise adjustment of the MFCs to achieve accurate gas flow rates. Further, a process with a pressure such as 30 torr, is more susceptible to gas phase nucleation (GPN).
Prior to the tungsten via fill process, a titanium nitride (TiN) film is reactively sputtered on the via sidewalls to act as a "glue" liner for the tungsten via fill. Alternative methods to deposit the "glue" liner include chemical vapor deposition or a combination of chemical and physical vapor deposition. The mechanism of the sputtering process is generally a line-of-sight process; therefore, the top area of the via is exposed more to the reactive species than the bottom of the via. The resulting profile includes an "overhang" at the top of the via as illustrated in FIG. 1. A via 100 comprises two sidewalls 102 and a TiN liner 103 with an "overhang" 104. The overhang 104 affects the nucleation film as the depositing tungsten generally grows away from the surface. In other words, the tungsten nucleation film growth is in a direction perpendicular to the surface of the sidewalls. As a result, the tungsten nucleation film will grow conforming to the existing overhang 104 from the TiN liner 103 and the top area of the via will close prematurely, creating a void.
The uniformity of a deposited film in patterned features like a trench or a via (such as the TiN liner 103) is referred to as "step coverage". The step coverage is typically determined by taking a ratio of the maximum thickness and the minimum thickness and converting the value to a percentage. By way of example, a 60% step coverage of the TiN liner 103 may indicate a maximum thickness of about 10 nanometers (nm) at the top of a via and a minimum thickness of about 6 nm at the bottom of the via sidewall. Ideally, the goal is to have 100% step coverage for a TiN liner deposition. As straight-sidewalls and high aspect ratios of the vias accompany the decreasing dimensions of integrated circuits, the problem of the step coverage for a TiN liner, such as the TiN liner 103 of FIG. 1, is increased. Substantial thinning of a TiN liner near the bottom of via sidewall is evident and illustrated with the TiN liner 103 of FIG. 1. The poor step coverage in the TiN liner 103, in turn, results in poor step coverage of the tungsten nucleation film. It is also conceivable that non-uniformity during the via etch process can result in areas without any liner coverage. The absence of any liner coverage reduces the growth rate of the tungsten nucleation film.
Solutions to resolve the voiding problem include varying the tungsten film chemistry, the tungsten nucleation film thickness, and the TiN liner thickness. The TiN liner thickness in comparison with the tungsten nucleation film thickness, however, is almost negligible. Typically, the liner thickness is about 10 nm and the tungsten nucleation film thickness is about 100 nm. The overhang profile created during the TiN liner deposition, therefore, is more pronounced after forming the tungsten nucleation film. By increasing the tungsten nucleation film thickness, the step coverage becomes worse as the overhang profile from the TiN liner is more pronounced. Depositing a tungsten nucleation film with a thickness that is too thin, on the other hand, forms a by-product called hydrogen fluoride (HF) which attacks the TiN liner and creates defects known as "woil holes" that results when the WF.sub.6 gas contacts the silicon. It is desired to fabricate semiconductor devices using reliable and robust processing methods to improve the tungsten fill characteristics for vias with high aspect ratios as the device structures continue to scale down below the quarter micron size.